Electronic sirens have found a high degree of acceptance in the marketplace, due in part to the flexibility of audio output available in their design. For example, the frequencies developed may be programmed to achieve an output highly perceptible to the human auditory response system. Further, inasmuch as the output of the devices is derived from an audio frequency electrical signal as opposed to mechanical devices, compact units are available which are ideally suited for vehicular installation.
Generally, the circuits utilized for the sirens include a signal generator which develops a pulse train, for example, a square wave output modulated in frequency and time envelope to achieve such special affects as "wail", "yelp" or "hi/lo". By way of further description, the yelp signal may be developed as a frequency sweep of about 650 H.sub.z to 950 H.sub.z carried out at a rate of three sweeps per second. Similarly, the hi/lo output represents a one second time envelope jump in frequency from 650 H.sub.z to 950 H.sub.z, while wail usually is developed as a four second frequency sweep in a range, for example of about 650 H.sub.z to 950 H.sub.z. As is known to those artskilled, generation of these outputs is a matter of somewhat straightforward electronics design, typical signal generators being described, for example, in U.S. Pat. Nos. 3,747,092; 3,504,364 and others.
Typically, the pulse train outputs are directed through a form of push-pull power transistor stages and an output or coupling transformer to a speaker, the latter components representing a reactive load. The speaker components of the systems essentially always are mounted for performance under somewhat rigorous environmental conditions. For example, when mounted upon vehicles they are subject to vibrations often of magnitudes causing short circuiting as a consequence of broken lead connections, the speaker coil being momentarily driven into the frame etcetra. When operated under inclimate weather conditions, whether mounted upon vehicles or the exteriors of buildings, rain and/or splashed water and snow may be driven by wind into the speakers, a situation again creating distinct possibilities for short circuiting phenomena to occur. These short circuits typically result in the destruction of the power transistor stages, the correction of which involves relatively high repair costs.
Generally, the power transistors are provided an excess base drive for the practical purpose of minimizing V.sub.CE SAT, to overcome gain variations in evidence in the individual output transistors and to accommodate for operational variations derived from temperature effects. Upon the occasion of a short across the load, the excess base drive will cause very large currents to flow through the power transistors. Inasmuch as transformer impedance is very low under short circuit conditions, collector voltages will approach supply voltage. The resultant voltage-current product then exceeds the region of safe operation of the transistors with the consequence of destroying them by forward bias secondary breakdown. Such destruction occurs in mere milliseconds, a rate far exceeding the operational protective capability of a conventional fuse. Resort to the use of a base voltage clamping network may occur to those art skilled as a solution to the above condition, however, the base-emitter voltage witnessed during operation of the power transistors is not sufficiently predictable nor is the characteristic slope thereof adequate for the operation of a fixed threshold clamp.
Whether occasioned by overload, short circuit phenomena or simply by operation within a hot environment, the power transistors will develop thermally induced leakage currents. Unchecked, these thermally generated base currents will tend to increase until a destructive phenomena termed in the art as "thermal runaway" is experienced. Further aggravating performance under adverse thermal conditions, the most desirable, compact electronic packaging configurations do not have a heat disipation capacity capable of accommodating thermal buildup at the power transistors. In the past, base connected impedance networks have been utilized to divert thermal-leakage currents, however, impractically low resistance values for the networks were required to accommodate all effects encountered. As another aspect of the utilization push-pull power transistor stages, it is important that the activation of these stages be in mutual isolation. In this regard, the drive utilized to turn on the transistors in required alternating fashion should be "non-overlapping", inasmuch as activation of both transistors simultaneously will evolve transformer currents which produce opposing magnetic fields. This produces a reduction in primary impedance during the overlap interval which will result in substantial current spikes at a time when the collector-emitter voltage is not minimum. Such conditions cause substantial heating which will slowly degrade the performance of the output or power transistors.